1. Technical Field of the Invention
This invention relates to radio telecommunication systems and, more particularly, to a system and method for adapting a time period for the collection of signal strength measurements from serving cells and neighbor cells prior to and during handoff of a mobile station in layered cell structures and across exchange boundaries.
2. Description of Related Art
In existing cellular radio telecommunication systems, all base stations have signal strength receivers that measure the current signal strength of signals on all frequencies utilized in neighboring cells. When a call connection between a mobile station and its serving base station deteriorates in strength and/or quality, the serving base station requests a handoff from the serving mobile switching center (MSC). Before handing off the mobile station, the MSC performs a locating function to locate or identify available neighboring cells that have adequate signal strength to satisfy the handoff request. Signal strength measurements are taken in the serving cell in which the mobile station is operating as well as in neighbor cells. The measurements are then collected and compared in order to determine whether the mobile station should be handed off and if so, to which neighbor cell.
Existing systems have a fixed timeout delay during which the MSC receives signal strength measurements. At the conclusion of the timeout delay, the MSC compares the measurements that it has received, and makes the handoff determination. The existing fixed timeout delay is independent of cell configurations even though different cell configurations may affect the time required to collect signal strength measurements from all of the neighbor cells. For example, if the serving cell is a microcell in a layered cell structure, additional time may be required to collect measurements from higher layered cells and neighboring microcells. In this situation, the fixed timeout delay may not be sufficient to collect all the signal strength measurements prior to processing the data, thereby excluding the possibility of considering higher layer cells or some neighboring cells as handoff candidates. The fixed timeout delay is not addressed by any so-called "Fast Handoff Algorithms" proposed within the cellular telecommunications industry for microcells. These Fast Handoff Algorithms deal with the measurement of signal strengths by a single signal strength receiving device as opposed to the collection of measurements from a plurality of cells.
Additionally, for situations in which the neighbor cells include cells within the serving cell's exchange as well as outer cells in cooperating exchanges, all signal strength measurements from cells within the serving cell's exchange are withheld from processing until either (1) measurements from cooperating exchanges are received, or (2) timeout from the signaling protocol used to communicate with the cooperating exchanges occurs. At that point, all received measurements are then processed. By the time conditions 1 or 2 occur, measurements from cells within the serving cell's exchange may be up to 13 or 14 seconds old, and may no longer be representative of signal strengths in those cells. However, the system cannot merely ignore the measurements from neighbor outer cells, because to do so would preclude inter-exchange handoffs. Therefore, a method is needed to balance between waiting for outer cell measurements and beginning handoff processing in order to optimize the handoff process.
Condition 1 may cause a handoff failure if one or more of the mobile switching centers (MSCs) in the cooperating exchanges is programmed with an excessive delay. An excessive delay may allow the mobile station to move out of the coverage area of the serving cell, causing the signal strength and/or signal quality to deteriorate to the point that the call is dropped before the measurements are processed and a handoff determination is made. MSCs in cooperating exchanges may be manufactured by different manufacturers who design their systems with different delays in returning signal strength measurements. Therefore, when the MSC in a cooperating exchange is manufactured by a different manufacturer, the delay in returning signal strength measurements is not within the control of the serving exchange, but nonetheless, may adversely impact its handoff performance.
Condition 2, waiting for timeout from the signaling protocol used to communicate with the cooperating exchanges to occur, may also cause handoff failures. This timeout delay may allow the mobile station to move out of the coverage area of the serving cell before the measurements are processed and a handoff determination is made. Some revisions of the IS-41 intersystem signaling protocol, for example, have had timeout delays as long as 15-seconds. This delay in processing the received signal strength measurements has particularly adverse effects when the mobile station is operating in a microcell and starts to move out of the serving cell. By the time the 15-second delay has elapsed, the mobile station may have moved out of the serving microcell, causing the signal strength and/or signal quality to deteriorate to the point that the call is dropped before the measurements are processed and a handoff determination is made. Therefore, in existing cellular radio telecommunication systems, the deployment of microcells must be restricted to avoid certain configurations having a higher probability of dropped calls due to the excessive timeout delay.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming, U.S. Pat. No. 5,301,356 to Bodin et al. (Bodin) discusses subject matter that bears some relation to matters discussed herein. Bodin discloses a system and method for ensuring that handoff requests take priority over new requests to engage voice channels. If no voice channels are available when a handoff request to a particular target cell is received, Bodin stores the handoff request in a corresponding queue for a predetermined period of time. If a voice channel becomes available while the handoff request is stored, the voice channel is utilized to satisfy the handoff request. Only if the handoff queue is empty are voice channels assigned to new call requests.
The predetermined time period of Bodin is separate and distinct from the timeout delay of the present invention. The predetermined time period of Bodin begins after the locating procedure is completed and has located neighbor cells which are acceptable as target cells for handoff, and after the handoff request is generated and stored in the handoff queue. At the conclusion of the time period of Bodin, the handoff request is removed from the queue. The purpose of the time period of Bodin is to ensure that if no handoff is possible to a particular target cell, each handoff request is directed to another target cell which may be able to satisfy the handoff request before the call connection deteriorates to the point that it is lost.
The measurement collection timeout delay of the present invention, on the other hand, is part of the locating procedure that identifies satisfactory target cells for handoff. As noted above, the timeout delay is the length of time that the MSC will wait for signal strength measurements to be received from serving and neighbor cells before it analyzes the measurements received to determine the best target cell. The adaptive queuing timeout delay of the present invention adapts the queuing timeout delay based upon network topology and the signal strength and quality in the target cell for handoff at the time the signal strength measurements were collected. This is a capability that is neither taught nor suggested by Bodin. Thus, review of the foregoing reference reveals no disclosure or suggestion of a system or method such as that described and claimed herein.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method for adapting the measurement collection timeout delay to different configurations of serving cells and neighbor cells in layered cell structures and across exchange boundaries. Such a system and method would adapt the measurement collection timeout delay for configurations in which the existing fixed timeout delay, the timeout delay of cooperating MSCs, or the timeout delay of intersystem signaling protocols increase the possibility of handoff failures. It would also be advantageous to have a system and method for adapting the queuing timeout delay for different network topologies. The present invention provides such a system and method.